The present invention relates to a safety device for the control of a web-fed rotary printing machine.
It has been known that tears or cracks in the paper web in the rotary printing machine, unless detected by fast acting devices for preventing damage to the machine, may become considerably expensive. Stopping and restarting of the machine due to the occurrence of such tears or cracks are also costly.
A device for avoiding web cracks, by controlling web tension in the roller rotary printing machines, has been disclosed in DE-PS 22 11 598. In this conventional device it has been recognized that the web tearing in the majority of cases is signalled by a change of the web tension whose frequency increases with the amplitude increase. The scanning of this web tension change, its indication and utilization for the control of the machine speed or web tension can prevent damage to the machine. This structure has the effect on the dynamic behavior of the paper web which tends to swing.
The disadvantage of this known device resides in that the web tears or cracks are not prevented when such tears or cracks occur in the regions of the edges of the web or in the regions of the glue connected web portions, specifically if the web is under relatively high quasistatic loading. A further disadvantage of the known device is that tears caused by the one-side pulling loads are not prevented either.
A device for preventing tears or cracks or similar damages to the advancing web has been disclosed in DE-PS 26 06 037. In this known device a mechanical scanning device has been provided, which includes a resilient key finger and means which alternate the key finger by penetrating into the crack or hole in the web and are connected to a known control means so that upon the occurrence of a tear or crack a control of the machine is possible.
The disadvantage of the above described device resides in that for contemporary roller rotary printing machines higher web speeds cause increasing inertia of the mechanically operated scanner which is susceptible to trouble and is very expensive for specific technical solutions.